The present invention relates to a refining process and an apparatus for metallic gallium (Ga), and it further refers to a high purity Ga suitable for obtaining a compound semiconductor such as a GaAs single crystal.
Among the compound semiconductors, the Group III-V compounds, particularly GaAs single crystals, are widely used as the substrates of electronic devices and optical devices such as high speed ICs and photoelectronic integrated circuits because they not only have superior high electron mobility which is about five times as high as that of an elemental semiconductor such as silicon, but also exhibit excellency in, for example, high frequency characteristics, magnetic conversion functions, photoreceptor functions and light emitting functions.
Wafers of GaAs single crystal are manufactured through various processes. The basic process thereof must comprise a step of growing GaAs crystals from a melt of Gaxe2x80x94As and a step of slicing them into wafers. A wafer (semi-insulating GaAs substrate) thus prepared is then subjected to selective ion injection or various types of epitaxial growth processes to finally obtain the desired semiconductor device element.
In using a GaAs single crystal (GaAs ingot) as a semi-insulating substrate, it is an indispensable requirement that the single crystal stably maintains a specific resistance (referred to hereinafter as resistivity) of 1xc3x97107 xcexa9xc2x7cm or higher. Although it is most desirable to obtain an intrinsic GaAs single crystal completely free from impurities and lattice defects, it is practically difficult to produce such an intrinsic GaAs single crystal of high purity because of the unavoidable crystal defects and residual impurities. As a reason for causing such difficulties, there can be mentioned the presence of impurities that accompany the raw material for Ga (gallium) used in the step of growing a GaAs crystal from the Gaxe2x80x94As melt.
In growing a GaAs crystal from a Gaxe2x80x94As melt, generally used is the LEC (Liquid Encapsulated Czocralski) process. This process comprises covering the surface of a Gaxe2x80x94As melt placed inside a crucible with B2O3, and pulling up a seed crystal of GaAs through the B2O3 layer while rotating the melt and applying pressure in an inert gas atmosphere. In carrying out this process, various improvements are devised to reduce the incorporation of impurities into the GaAs single crystal as much as possible, such as using a crucible made of PBN (Pyrolytic Boron Nitride) or controlling the gaseous atmosphere.
In spite of the improvements made on the constitution of the apparatus and on the process conditions, the probability of incorporating the impurities into the GaAs single crystal increases if the concentration of impurities incorporated in the starting melt from which the GaAs crystal is grown remains high. That is, it is still difficult to obtain a high quality GaAs single crystal if the purity of the raw materials for Ga and As remains low. Among the impurities which accompany the raw materials for Ga and As, there certainly are impurity elements having a low segregation index that are less incorporated into the growing crystal and reside in the melt; however, from the viewpoint of improving the yield in producing GaAs single crystals, it is still undesirable to result in a melt containing impurity elements at high concentration. Accordingly, the concentration of impurities in the raw materials for Ga and As is preferably as low as possible and it is further desirable to previously recognize the type and content of each impurity present in the raw material.
Concerning the raw materials for Ga and As for use in producing GaAs single crystals, it is relatively easy to find a commercially available high purity As (arsenic) having a purity of 7N (seven nines; stands for a 99.99999% purity, and is sometimes used hereinafter to express the purity). However, the case for raw Ga materials is not so simple. Any raw Ga material contains, to some extent, a variety of impurities in various forms depending on its origin, and, the quantity of the impurities fluctuates in general. It is therefore difficult to stably obtain a raw Ga material free from impurities which are inconvenient for the production of GaAs single crystals. Furthermore, with the present day analytical technology (glow discharge mass spectrometer) for analyzing the content of the impurity elements present in metallic Ga, it is difficult to obtain reliable results for each of the components incorporated at a level of 0.01 ppm or lower. It can be understood therefrom that it is even difficult to know the exact concentration of each of the impurity elements contained in trace quantities in the raw Ga material to be used for the production of GaAs single crystals.
In addition to the aforementioned GaAs single crystals, compound semiconductors using Ga include GaP, GaN, etc. Because a GaP single crystal has excellent photoreceptor and light emitting functions, it is used as a substrate for optical devices such as light emitting devices. A GaP single crystal wafer is produced by first synthesizing a polycrystalline GaP, pulling up the polycrystalline GaP as a GaP single crystal and by means of a process similar to, for example, the aforementioned LEC process, and slicing the resulting GaP single crystal ingot. A light emitting device can be finally obtained by performing liquid layer expitaxy. To obtain a light emitting device of high luminance in this case, the incorporation of impurities in the GaP single crystal substrate must be suppressed to the lowest limit. Particularly harmful are the impurities which increase the concentration of the carriers on synthesizing the polycrystalline GaP and lowers the resistivity. Similar to the case of GaAs, furthermore, the incorporation of such harmful impurities is believed to be originated from the raw Ga material in many cases.
As a process for refining metallic gallium to remove impurities from the raw materials, conventionally known processes include acid processing, electrolytic smelting, zone melting, pulling up crystals, recrystallization by melting and solidification, etc. Among these processes, the recrystallization process comprising melting and solidification is advantageous in that it enables refining using a relatively simple installation and operation. In solidifying a liquid of a raw gallium material containing an impurity, there is known a phenomenon as such that the impurity concentration of the crystal becomes lower than that of the residual liquid. The principle of this process is based on this phenomenon.
For the process of refining gallium by utilizing the phenomenon above, proposals for improving the process conditions and operations can be found in, for example, JP-A-Sho62-270494 (the term xe2x80x9cJP-A-xe2x80x9d as referred herein signifies xe2x80x9can unexamined published Japanese patent applicationxe2x80x9d), JP-A-Sho63-242996, JP-A-Hei2-50926, JP-A-Hei2-50927, JP-B-Hei2-53500 (the term xe2x80x9cJP-B-xe2x80x9d as referred herein signifies xe2x80x9can examined published Japanese patent applicationxe2x80x9d) JP-A-Hei6-136467, etc.
At present, in producing compound semiconductors such as GaAs and GaP, it is practically impossible to obtain a highly pure metallic gallium having a purity of 6N or 7N or even higher and also provided with reliable analytical data for each of the impurity contents. Since this caused a problem in producing high quality compound semiconductors such as GaAs and GaP, a first object of the present invention is to overcome this problem.
Among the prior art technologies for producing high purity gallium, the recrystallization process using melting and solidification comprises separating the crystalline gallium (solid phase) containing impurities at a low concentration level from the residual liquid (liquid phase) containing impurities at a higher concentration, and is based on the concept of separating the solid phase from the liquid phase differing in impurity concentration. In order to separate the high purity solid phase from the liquid phase, it is necessary to separate the.solid phase when the residual liquid is still present at a relatively large quantity. This inevitably results in a low yield of high purity gallium.
For instance, in JP-A-Hei6-136467 is disclosed a process which comprises inserting a cooled tube into the central portion of the molten raw gallium material placed inside a vessel, thereby allowing solid gallium to precipitate on the surface of the tube, and then pulling up the resulting tube having thereon the precipitated gallium. The disclosure teaches, however, that it is preferred to complete the operation of precipitation at a stage that the solidification ratio (the ratio of the precipitate) accounts for 30 to 40%, and that even under the most controlled conditions, the solidification ratio attainable is in the range of 60 to 70%. Accordingly, this results in a large quantity of gallium remaining in the liquid phase, and this limits the yield of the refined product.
The recrystallization process using melting and solidification is rarely adopted as a mass production technology to produce a high purity gallium having a high purity in the level of 6N or 7N in an industrial scale due to its poor controllability and productivity.
Accordingly, a second object of the present invention is to establish a process for producing high purity gallium at high yield and with superior controllability.
Furthermore, a third object of the present invention is to provide, for a high purity metallic gallium for use in the preparation of single crystals of compound semiconductors such as GaAs and GaP, a means to recognize the concentration of each of the impurities that are unable to reliably quantify their concentration by the existing analytical method using glow discharge mass spectrometer; at the same time, it is also the object of the present invention to provide a high purity gallium containing such impurities in a trace level, but with given approximate concentration.
According to an aspect of the present invention, there is provided in a process for separating impurities from a raw gallium material containing impurities, a process for refining gallium comprising progressively solidifying a raw gallium material provided in a liquid state inside a vessel while applying stirring, such that the diameter of the tubular solidification boundary gradually advances towards the center of the vessel to reduce the diameter of the tubular solidification boundary, and separating the liquid phase remaining in the central portion of the vessel from the solidified phase before the entire raw material inside the vessel is solidified.
In the process above, the stirring can be applied by a magnetic field, and particularly, the stirring is preferably applied by a magnetic field in such a manner that a circular flow is generated in the liquid phase in the circumferential direction.
In the aforementioned refining process, the solidified phase inside the vessel can be molten again after separating the liquid phase remaining in the central portion of the vessel from the solidified phase, and by repeating the process above, the purity of the solidified phase can be progressively increased. In this case, the solid phase is preferably reserved as a seed crystal on the inner wall plane of the vessel on melting the solidified phase.
As an apparatus for performing the refining process above, there is provided an apparatus for refining gallium comprising a vessel having a cylindrical inner wall, a cooling zone attached to the outer peripheral plane of the vessel, a heating zone provided on the inner side of the inner wall of the vessel, a suction pipe installed at the central portion of the vessel, and a magnetic rotator placed on the lower side of the vessel. Furthermore, as another apparatus for performing the refining process above, there is provided an apparatus for refining gallium comprising a vessel having a cylindrical inner wall, a cooling and heating zone attached to the outer peripheral plane of the vessel, a suction pipe installed at the central portion of the vessel, and a magnetic rotator placed on the lower side of the vessel. In these apparatuses, a heating zone can also be provided at the bottom portion of the vessel and to the outer periphery of the suction pipe.
In addition to the aforementioned processes and apparatuses for refining gallium, an aspect of the present invention provides a high purity raw Ga material for use in the preparation of a compound semiconductor, characterized by a raw Ga material used for preparing a compound semiconductor which yields a difference xcex94C=|xcexa3Anxe2x88x92xcexa3Bn| of 5 ppm by atomic or lower when subjected to a xe2x80x9ctest method for impurity-concentrated Gaxe2x80x9d as defined below, where xcexa3An represents the total quantity of the components contained in the sample of an impurity-concentrated Ga, which is at least one element of group A components selected from the group consisting of B, Na, Mg, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Au, Hg, Pb, and Bi; and xcexa3Bn represents the total quantity of the components contained in the sample of impurity-concentrated Ga, which is at least one element of group B components selected from the group consisting of F, Si, S, Cl, Ge, Se, Sn, and Te. xe2x80x9cTest method for impurity-concentrated Gaxe2x80x9d is defined as a test method comprising:
using an apparatus for refining gallium comprising a vessel, having a cylindrical inner wall made of a 3 mm thick SUS304 steel sheet provided with a 0.3 mm thick fluororesin coated inner wall plane, the vessel having an inner radius of 60 mm and a height of 40 mm, a cooling zone attached to the outer peripheral plane of the vessel, a suction pipe installed to the central portion of the vessel, and a magnetic rotator provided to the lower portion of the vessel;
filling the vessel with a raw Ga material in liquid state at a quantity as such that it amounts to 30 mm in height inside the vessel while purging the space inside the vessel with an inert gas; and
obtaining a sample of impurity-concentrated Ga as follows:
while applying a circular flow of 100xc2x110 rpm to the liquid raw Ga material by using the rotator, maintaining the liquid raw Ga material at a temperature of 29.6xc2x10.5xc2x0 C., and passing a cooling water at a temperature of 5xc2x0 C. through the cooling zone, thereby allowing progressive solidification of the liquid to proceed from the inner wall of the vessel towards the central portion of the vessel at a solidification rate as such that the entire liquid may solidify in 60xc2x15 minutes, then sampling the liquid phase through the suction pipe described above when the radius of the remaining liquid phase becomes 20 mm.